1. Field of the Invention
The present invention relates to an ink jet recording apparatus, an ink supplying mechanism and an ink jet recording method in which ink is ejected from an ink jet head while the ink is circulated.
2. Description of the Related Art
An ink jet recording apparatus and an ink jet recording method have been known in which ink is ejected from a nozzle of an ink jet head while the ink is circulated. In such an ink jet recording apparatus, leakage of the ink from the nozzle and suction of air from the nozzle are prevented and a proper ejection droplet shape of the ink must be provided. To realize these, it is considered desirable that the pressure near the nozzle of the ink jet head should be maintained at a proper value. For example, in an ink jet recording apparatus as shown in FIG. 11, the ink tank is arranged below the head in order to realize a negative pressure near the nozzle. The head is connected to a lower ink tank part via a duct. In the case where the liquid surface in the lower ink tank part is situated below the surface of the nozzle plate by a height h, the potential pressure to the vicinity of the nozzle in the ink chamber is −ρgh (where ρ is the density of the ink, and g is the acceleration of gravity). This liquid surface is opened to the atmosphere. Therefore, when the pressure loss in the ink duct is sufficiently small, the vicinity of the nozzle in the ink chamber is maintained at the negative pressure of −ρgh.
However, there often is a mechanical limitation of a printing machine in supplying the ink from the position below the head as described above. For example, generally, in a serial-scan printing machine, a scanning mechanism including a belt and a slider exists near the head, and it is divided at the head into an upper part and a lower part. Also, in a fixed-head printing machine, generally, the ink jet head ejects ink downward and a print sheet moves horizontally below the head. Therefore, the printing machine is structurally divided into an upper part and a lower part by the print sheet and its feed mechanism. If ink is to be fed to the head from the ink tank situated below the head in such a printing machine, the ink duct is most likely to be long and meandering. Therefore, it is also difficult to secure the diameter of the duct.
With a narrow, long and meandering duct, the increased flow path resistance cannot be ignored. Therefore, the negative pressure in the pressure chamber near the nozzle is changed by the quantity of ejected ink affected by the flow path resistance, and it becomes difficult to maintain a proper negative pressure.
Also, a long and meandering duct complicates the structure of the printing machine and causes poor maintainability. Since the ink volume in the duct is large, waste of ink increases.
For a serial-scan low-speed printing machine, a technique is provided that includes a mechanism for generating a negative pressure, formed by a porous member, deformative bag or the like above the vicinity of the head. However, with these mechanisms, it is difficult to secure compatibility with various types of ink. Also, no idea is given of applying this technique to an ink supply system of a circulation-type head.
For a large-size printing machine or the like, a technique is provided in which a sub-tank supplied with negative-pressure air is installed above the vicinity of the head, and ink is pumped up from the main tank to the sub-tank by a pump, enabling installation of the sub-tank near the head. Therefore, the pressure loss in the duct from the sub-tank to the head can be reduced relatively easily, but no idea is given of applying this technique to a circulation-type ink supply system.